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Re: Group velocity for Cherenkov photon propagation in G3/G4 [message #11459 is a reply to message #11457] Wed, 26 January 2011 14:12 Go to previous messageGo to previous message
Jochen Schwiening is currently offline  Jochen Schwiening
Messages: 84
Registered: June 2009
Location: GSI, Darmstadt
continuous participant
From: *gsi.de
Hi Stefano,

thanks for the quick response.

I know from past work with (standard, non-Panda) Geant4 that
G4 does in fact use the wavelength-dependent photon group
velocity to calculate the propagation time. It uses the
formula n_group = n_phase - lambda*(dn_phase/dlambda).
It even comes out at the correct values, about 19.1cm/ns for
300nm photons, 20.3cm/ns for 600nm photons.
My quick and dirty check in our Panda simulation data finds
almost constant values of 19cm/ns +-0.1ns/cm for the entire
wavelength range from 300-650nm.

I don't think that I know how to find out in ProcessHits in
PndDrc.cxx how far the photon traveled since the path is
defined by the many internal reflections. We instead calculate
the path from the photon production angle and then calculate
the velocity from this path and the photon propagation time.
It's possible that this code has a bug and that's why I would
like to debug the (group) velocity of the photon in the medium
using something like the GetVelocity function but I don't see
that in our VMC environment.

Any other (simple) way to check the photon speed?

Thanks,
Jochen
 
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